The present invention relates to a thermal compensated interconnect apparatus for circuit boards and to the method of making such interconnect apparatus, and in particular to such an interconnect apparatus for surface mounted circuit technology.
The development of solid state circuitry in the electronics industry has included a continuous increase in the density of components, simultaneously with the miniturization of components. Printed circuit boards are standard in the electronic industry. The circuit board includes an insulating substrate having circuit connectors and certain passive components printed on at least one side. The circuit board is further formed with very precisely located pin holes defining interconnect points for interconnection of circuit components to the printed components and conductors. Other components are mounted to the board with component leads projecting through connection locating pin holes. Circuit connections are made through the pin holes to the printed conductors by soldering to the back side of the circuit board at the respective pin holes. Although such circuit board technology is standard and with the available significant miniturization has increased component density, it has inherent limitations. Generally only one side of the circuit board is available for circuit component mountings. More significantly, the manufacturing process is relatively expensive because of the precision requirements in the forming of the circuit board with the interconnect pin holes and the like. A more recent development includes surface mounted technology wherein the components are mounted directly on a given printed substrate surface, with a direct interconnect between the printed conductors and associated printed components and separate surface mounted components. The surface mounted technology permits a significant increase in the component density as well as reduced cost. Even though surface mounted technology has several advantages, the interconnection of the main incoming power terminal to the surface mounted circuit has presented a particularly difficult problem. The main leads and terminals require relatively heavy interconnect conductors to accommodate the necessary current and voltages. The current loads, particularly with the miniaturization of the circuit components introduces thermal factors which must be considered in the system design. The art has recognized that the interconnect apparatus to the circuit must compensate for relative thermally induced movement of the interconnect apparatus between the circuit board terminals and the lead frame terminal. The prior art has generally used manual wiring of appropriate compensating conductors between the circuit board terminals and the lead frame terminals. The manual systems are not only costly but highly dependent on the skill of the operator to maintain appropriate quality standards. Alternatively, relatively sophisticated and costly automated systems have been suggested and used. For example, General Motors, in its manufacturing systems, have developed automated systems for placing and interconnecting of a compensating lead which is soldered to the circuit board and welded to the main frame lead. Fairchild Industries, Inc. in the manufacture of electronic assemblies uses a strip conductor for interconnecting of an external terminal to a circuit board base conductor. Generally, a plurality of interconnect elements are joined by common mounting strip and aligned in a linear fashion on the circuit board. Each of the individual interconnect strips are formed into a generally offset configuration with a small tab portion adapted to be aligned with the external aligned terminals and a 90.degree. offset base portion adapted to be aligned with a significantly larger solder area on the circuit board. The terminal tab and the circuit board base are interconnected by a relatively large integral curved portion to introduce flexibility into the interconnect element for accomodating thermal conditions and the like. After placement, the mounting portion is cut away to establish the desired individual interconnections. Although the final interconnect provides a thermally compensated connection, the manufacturing process does not appear to be adapted to versatile automated processing. The orientation of the interconnect structure is quite confined by the alignment concept. In many applications the electronic circuit board may not be particularly adapted to produce the alignment of the several terminals in one line as required by strip-type assembly. Rather, a more individualized placement and completion of the interconnect elements is desirable in a production facility for forming of widely differing electronic circuits. The solder connection to the circuit board is generally required to permit the low temperature interconnection required by the sensitivity of the circuit board component and the fragile circuit board connections. The lead frame connection is spaced from the circuit board connection and is more fully adapted to a higher temperature welded connection. Where the sophisticated machinery or system heretofore suggested is not economically feasible, the manual wiring and soldering connections are more widely used. Further, the lead wires used in the interconnect are often fragile and may be subject to short life in the field.
These as well as other difficulties and the highly developed state of the pin-hole connection systems have significantly limited implementation and use of surface mounted technology in the electronic arts. There is therefore a significant need and demand for a relatively low cost, automated interconnect apparatus and method which can maintain a high quality interconnect in the normal environment encountered by modern electronic circuitry.